Electron tube circuit



Dec. 26, 1950 HEATON 2,535,285

ELECTRON TUBE CIRCUIT Filed Sept. 14, 1945 4 I OUTPUT u 28 1 30 I 26INPUT NEG. BIAS NEG. BIAS. NEG. BIAS INVENTOR. ROY G. HEATON am/WM LAMATTORNEY.

Patented Dec. 26, 1950 ELE CTRON TUBE CIRCUIT Roy G. Heaton, Cambridge,Mass., assignor, by mesne assignments, to the United States of Americaas represented by the Secretary of War Application September 14, 1945,Serial No. 616,381

6 Claims. 1

This invention relates in general to pulse generators and moreparticularly to an electrical circuit for generating a relatively strongoutput pulse of long duration. The pulse, for example, may be used forblanking a receiver.

In radio detecting and ranging apparatus and in television, it issometimes very important to have a circuit able to generate pulses ofrelatively large amplitude and long duration. The circuit should becapable of being triggered by a relatively weak trigger pulse. It istherefore an object of this invention to provide a pulse generator whichcan be readily triggered and which will produce pulses of relativelylong duration and large amplitude.

One means for obtaining pulses of this shape is the blocking oscillator.The blocking oscillator in this invention is triggered by smallrelatively weak pulses which need not have a special shape. Theproduction of such trigger pulses is relatively simple and may beaccomplished by any one of several well known means. In the preferredembodiment of this invention, the trigger pulse is supplied from anearby radio transmitting apparatus through a coaxial cable and theblocking oscillator output pulse is used to blank a receiver associatedfor instance with a beacon. This is necessary, for example, where abeacon is in the vicinity of radio transmitting apparatus and it is notdesired to have the beacon respond to that transmitter. If desired, thetransmitted pulse may be picked up by means of a directional antennawhich can be pointed toward the transmitter which is not desired totrigger the beacon.

Another object of the invention is to devise a simple blockingoscillator circuit, using standard tubes, which will produce a strong,well shaped pulse of relatively long duration.

A still further object is to provide a blocking oscillator with an inputamplifier to permit the use of weak trigger pulses.

Other objects and advantages will become readily apparent from thehereinafter described specification.

The invention in general contemplates the use of two vacuum tubes usingthe primary winding of a transformer as a common plate impedance. One ofthe secondary windings is connected to the grid of the first tube insuch a way that an increase in primary current places a strong positivebias on the grid. A second secondary Winding is connected to the cathodeof said first tube in such a way that when the primary currentincreases, the cathode becomes more positive. The output voltage pulsefrom resistor the first tube is taken from a resistor in series with thecathode. The output pulse produced by the blocking oscillator isrelatively long due to having windings in both the grid and cathodecircuits. The duration of the pulse may be changed by adjusting thecapacitance of a capacitor between the cathode and grid circuit or theresistance of a resistor connected between the grid circuit and ground.The action of this blocking oscillator is initiated by a relatively weakpositive pulse on the grid of the amplifier tube, which acts as anamplifier for the pulse. The pulse causes a sudden increase in platecurrent of the amplifier tube. This sudden increase in current throughthe primary of the transformer induces a pulse of voltage in each of thesecondary windings. This pulse in the secondary windings will change thebias on the blocking oscillator tube in such a way as to increase itsanode current and initiate the blocking oscillator action.

The invention will be best understood by referring to the simple figurewhich shows one embodiment of the invention.

The figure shows a terminal I to which a pulse is applied. Terminal l isconnected, through capacitor 2, to the grid of tube 3. The grid of tube3 is connected through resistor 4 to a voltage divider, consisting ofresistors 5 and 6 connected between ground and a source of negativepotential. This places a negative bias on tube 3. The cathode of tube 3is connected directly to ground. The anode of tube 3 is connected to asource of positive potential through the primary winding 1 ofatransformer 8. The anode of tube 9 is also connected through winding 1in said source of positive potential. A secondary winding l0 has oneterminal connected to the grid of tube 9 and the other to a H. The otherside of resistor II is connected to a voltage divider consisting ofresistors l2 and I3 connected between ground and a source of negativepotential. This places a negative bias on tube 9. Common junction of thetwo resistors is bypassed to ground through capacitor [4. The cathode oftube 9 is connected to ground through a secondary winding 15 of thetransformer 8 and a resistor 16. The resistor I6 is bypassed bycapacitor ii. The purpose of capacitor I1 is to prevent an initial veryshort negative pulse at point I9. The negative pulse is very undesirablewhen the main pulse is used to blank a receiver. Capacitor I8 isconnected between the cathode of tube 9 and one terminal of the winding[0.

The positive output pulse of the blocking oscillator is taken betweenpoint l9 and ground. This pulse is applied to the cathode of tube 28.The grid of tube 20 is connected to an adjustable potentiometer ZIthrough a resistor 22. The adjustable potentiometer is connected betweenground and a source of negative potential to provide a variable negativebias for the grid of tube at. The anode of tube 20 is connected to asource of positive potential through resistor 23. The output is takenoff through capacitor 24. The signal applied to the grid of, tube 28 istaken, through capacitor 25, from the anode of tube 25. The anode oftube 26 is connected to a source of positive potential through resistor27. The grid of tube 26 receives the signal through capacitor 28. Thegrid is connected to ground through resistor 29. The receiver input isconnected to terminal 30.

The positive trigger pulse, which may be any shape, is applied toterminal and through capacitor 2 to the grid of tube 3, which serves to,amplify the pulse. The grid is held at a negative potential by beingconnected to the voltage divider through resistor The positive pulsecauses tube 3 to draw a pulse of anode current through winding 5 of thetransformer. This causes a pulse of voltage to be induced in transformerwindings lo and The polarity of these windings is so arranged that boththe grid and the cathode of tube a receive positive pulses. Point 3!,the junction of capacitor l8 and resistor H, is tied to the cathode oftube 9' through capacitor i8. Point will therefore initially rise inpotential substantially as much as the cathode. The positive pulseapplied between point iii: and the: grid produces a change in bias ontube a which causes its anode current to increase through the winding land thereby initiates the blocking osciilator action. The grid is veryrapidly driven sufficiently far positive so that grid current flows,charging capacitor iii in such a manner that point 3! will be much lesspositive than the cathode. The positive pulse onthe grid also causes theanode current to increase very rapidly in the typical blockingoscillator regenerative action. Capacitor I8 is also charged, with thesame polarity, by current flowing from the cathode through thecapacitor, through resistors M, 2 and i5. and winding 15.

The anode current of tube 9 will continue to increase very rapidly untilsaturation takes place. The saturation may be either in the tube or thetransformer or both. When saturation takes place the current remainsrelatively constant for a period of approximately 2 to 25 microseconds,depending on the tube, the value of capacitor I 8 and the value ofresistor it. At the expiration of this time, the degenerative action ofthe blocking oscillator takes place and the current very rapidlydecreases. The duration of the pulses can be varied by changing thevalue of capacitor i8 and resistor H.

The output voltage prlse from the blocking oscillator is taken acrossresistor it. This pulse may be applied directly to the cathode ofanother tube without an intermediate blocking capacitor. This permitsalmost instantaneous recovery of the blanked tule after the "lan"'ngpulse.

The use of resistor iii in i 1 the cathode to provide the: pulsesubstantially ehrninates overshoot in the output pulse.

The cathode of tube connected to point l9 to receive the blanking pulse.The grid of tube 2a is connected to an adjustable potentiometer ii toprovide a variable source of negative bias for the tube. The positivepulse on the cathode of tube 2e makes the bias on the tube sufllcientlynegative to completely out off the tube. Tube 22 is coupled to tube by acon ventional resistance-capacitance coupling. The input signal isapplied to the grid of tube 26 through blocking capacitor 28.

The type of tube used is not critical. Two tube types which are wellsuited for use in this circuit. are. the 7P8 and the 5SN7. These tubesare mentioned merely by way of example and it is not intended that theinvention should be considered limited to these specific tubes or evento the class of twin triodes in general.

The following typical values are given for the components of theblocking oscillator portion of the circuit using a type 7F8 tube:

Resistor H ohms 1,000 Resistor i2. do 15,000 Resistor l3 do 100,000Resistor it do- 1,000 Capacitor him"; microfarads .00925 Capacitor i! do.00025 Capacitor i8 do .1

Using the above listed values with volts as the source of negativepotential and 250 volts as the source of positive potential, a pulse ofapproximately 9 microseconds was obtained.

The aforementioned values are given solely for illustrative purposes andare not intended to limit the invention in any Way.

It will be apparent that there may be deviations from the invention asdescribed. which still fall fairly within the spirit and scope of theinvention. For example, the output pulse of the blocking oscillator neednot be used for blanking a tube but could be used any place where apulse of relatively long duration is desired.

Accordingly, I claim all deviations which fall fairly within the spiritand scope of the invention as identified in the hereinafter appendedclaims.

What is claimed is:

1. A blocking oscillator comprising a vacuum tube having an anode, acathode and at least one grid, a transformer, a first winding of saidtransformer for electrically connecting the anode of said tube to asource of positive potential, a second winding of said transformer forelectrically connecting a grid of saidtube to a source of negativepotential, a third winding of said transformer electrically connected tothe cathode of r said tube, a capacitor electrically connecting thecathode of said tube to said second winding, and resistor meanselectricallyconnecting. said third winding to a source of referencepotential.

2. A blocking oscillator as set forth in claim 1 in which the commonjunction of said third winding and said resistor means is electricallyconnected to the cathode of a second tube.

3. In a blocking oscillator, a vacuum tube having an anode, a cathodeand at least one grid, a transformer having a primary winding forconnecting said anode to a source of positive po-ten tial and at leasttwo secondary windings, a first secondary winding for connecting a gridof said tube to a source of negative potential, a second secondarywinding connecting the cathode of said tube to a source of referencepotential, 2. capaci tor means connecting said cathode to said firstsecondary winding, and resistor means connected between said secondsecondary winding and said source of reference potential.

4. A circuit for producing a blocking pulse comprising at least twovacuum tubes each having an anode, a cathode and at least one grid, atransformer, a first winding of said transformer for electricallyconnecting the anodes of said tubes to a source of positive potential, asecond winding of said transformer for electrically connecting a grid ofthe first tube to a source of negative potential, a third winding ofsaid transformer electrically connected to the cathode of the firsttube, a capacitor electrically connecting the cathode of said first tubeto said second winding, and resistor means electrically connecting saidthird winding to a source of reference potential.

5. A circuit for producing a blocking pulse as set forth in claim 4 inwhich the common junction of said third winding and said resistor meansis electrically connected to the cathode of a third tube.

6. In a blocking oscillator, a vacuum tube having at least a cathode, ananode and a grid, a transformer having a primary winding and twosecondary windings, a source of positive poten- REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,292,835 Hepp Aug. 11, 1942 202,358,297 Bedford Sept. 19, 1944 2,396,439 Schlesinger Mar. 12, 19462,411,573 Holst et a1 Nov. 26, 1946

